Nikola is a static website and blog generator. The very short
explanation is that it takes some texts you wrote, and uses them to create
a folder full of HTML files. If you upload that folder to a server, you will
have a rather full-featured website, done with very little effort.

Its original goal is to create blogs, but it supports most
kind of sites, and can be used as a CMS, as long as what you present to the
user is your own content instead of something the user generates.

Since Nikola-based sites don't run any code on the server, there is no easy
way to process user input in forms.

Keep in mind that "static" doesn't mean boring. You can have animations,
slides or whatever fancy CSS/HTML5 thingie you like. It only means all that
HTML is generated already before being uploaded. On the other hand, Nikola
sites will tend to be content-heavy. What Nikola is good at is at putting
what you write out there.